1. Field of the Invention
This invention relates to security systems, specifically security systems which employ video equipment for motion detection. Disclosed is a system which allows for motion detection of selected areas within the field of view of a video camera. This system contains an integrated control system which dynamically and automatically updates the motion detection mask to correspond to alternative views, based upon a predetermined set of criteria associated with such views.
2. Discussion of the Related Art
Video systems are well known in the field of security systems. In a typical security system, one or more video cameras are placed so as to provide a field of view of the area under surveillance. These video cameras convert a visual image into an electronic form suitable for transmission. A control station, either co-located within the surveillance area or remote from the area, receives the signals from these cameras and displays the video image at a console, for security assessment and recording. Typically, a person monitors the images from the cameras on a video screen and initiates security measures if the received image indicates unauthorized activities. Often the monitoring person (hereinafter the monitor) is responsible for monitoring the images from multiple cameras simultaneously, and means are provided to assist in this process.
Automated motion detection systems are employed to alert the monitor of the presence of activity within the view of a camera, as typified in U.S. Pat. No. 4,458,266. These motion detection systems operate by detecting changes in the sequential electronic images of the same scene. A change in the scene implies the entry or exit of an item from that scene. When a change is detected, an alarm is sent to the monitor for a security assessment. The monitor will view the sequence of images which caused the alarm, as well as other images, from this camera or others, to determine whether the alarm requires the initiation of security measures such as notifying the police or activating a warning signal. These motion detection systems can be co-located with the camera, or remote from the camera. They are often co-located with the camera and operate so as to transmit the images to the control station only in the event of an alarm, thereby saving communications bandwidth and costs.
These motion detection systems are highly effective in areas within which little or no activity is expected to take place, for example, in office buildings or parking lots after business hours. Motion detection systems are not as effective in the security of areas within which activity is normally expected, because the number of alarms which will be sent will overburden the monitor.
Within a field of view of a camera, there are often areas within which minimal activity is expected to take place, and other areas of higher activity. For example, within a bank, one would expect activity at the counter, but might expect minimal activity at the entrance to the safe. It would be desirable to send an alarm, or more descriptively, an alert, to the monitor each time activity occurred at the entrance to the safe, but not each time activity occurred at the counter. A separate camera, with a limited field of view, may be dedicated to the view of the entrance to the safe to provide this selective security. A motion detector attached to the camera would alert the monitor of activity near the safe. Alternatively, sections of the received image of a wider field of view could be purposely excluded from the motion detection system, also disclosed in U.S. Pat. No. 4,458,266. This purposeful exclusion of portions of the image is termed herein as masking. A camera with a view of the bank counter and safe could employ a motion detector with the bank counter masked out. The picture elements (pixels) of the video image corresponding to the area to be excluded from motion detection are blacked-out, or masked, so that subsequent images within this area remain constant, regardless of the actual activity as seen by the video camera. For this masking of selective portions of the video image to be effective, the correspondence between the masked pixels and the excluded area must be maintained. Typically, the mask is created when the camera is initially positioned, blanking out the selected areas within the camera""s fixed field of view. A change of environment, such as a remodeling of the secured area, or repositioning of the camera, requires the creation of a new mask for the changed view.
Also common in security systems are cameras with adjustable fields of view. Such security systems allow the monitor to adjust the camera""s field of view to better assess the situation. The adjustment of the camera""s field of view is conventionally described in terms of pan (adjustment in the horizontal direction), tilt (adjustments in the vertical direction), and zoom (adjustment of the magnification of the image). Conventionally, systems which provide for pan, tilt and zoom cameras (hereinafter P/T/Z cameras) are incompatible with motion detection systems. A change of the camera field of view will be detected by most motion detection systems as a detected motion, because the motion detection system does not distinguish between an image change caused by movement of the camera or by movement within the camera""s field of view. Tradition P/T/Z camera systems, if equipped with motion detection systems require the monitor to disable the motion detector prior to the pan/tilt/zoom movement, then to reenable the motion detector when the movements are complete. Experience has shown that monitors commonly forget to reenable the motion detector, obviating their effectiveness as a security device.
P/T/Z camera systems are further incompatible with selective area motion detection, as described above, because a pan/tilt/zoom movement changes the camera""s field of view, invalidating the correspondence with the installed motion detection mask. For selective area motion detection, an adjustment to a camera""s field of view requires a corresponding adjustment to the motion detection mask. Typically, this requires the creation of a new mask, specific to this new field of view. Because the creation of a new mask is a time consuming process, and because a P/T/Z camera has an infinite number of possible fields of view, each requiring a different mask, selective area motion detector systems rarely employ P/T/Z cameras.
An automated adjustment of a mask, to correspond to a change in the camera""s field of view cannot be made unless the adjustment of the camera""s field of view is integrated into the motion detection mechanism. Conventionally, the continuous recomputation of a mask, as the camera is adjusted, requires a continuous awareness of the changes in the camera""s field of view, in terms of the angular rotation through the horizontal and vertical axes, as well as changes in the lens magnification factors. Such a computation would involve complex coordinate transformations, as well as continuous feedback from the camera related to its field of view parameters. Such a system would be extremely costly, if at all practical.
It is the purpose of this invention to provide a system which provides for selective area motion detection using conventional pan/tilt/zoom camera technology, without requiring complex coordinate transformation techniques.
This invention is premised on the observation that, even though a P/T/Z camera may have an infinite number of fields of view, a select subset of views are often sufficient to provide for the security of the entire area under surveillance. Further, although the P/T/Z camera may be continually changeable, by a continuous activation of the pan, tilt, or zoom controls, the camera is typically stationary when motion detection is utilized.
In accordance with this invention, at each select, stationary, field of view, a mask image corresponding to this view may be downloaded into the mask of the motion detector. A set of masks, each corresponding to a select field of view, are stored, or computed, at the monitor station, and downloaded as required. Any and all processing to create each specific mask image need not be performed in real time, as the camera is moving, nor need it be performed after each P/T/Z camera movement. By providing for the Is loading of a mask corresponding to a particular view, the motion detector system can be one with minimal capabilities, and hence, minimal costs. The effectiveness, and cost, of the system will be determined merely by the number of select views for which a mask is required. In a preferred embodiment, a fixed set of predefined views, each with a predefined mask, are created and stored at the monitor station when the system is deployed, and thereafter as the environment may change. The monitor need merely choose one of these predefined views as a target view to effect selective area motion detection utilizing the target view""s predefined mask.
The selection of a predefined mask, or the computation of a new mask, still requires that the mask correspond to the camera""s particular field of view. This invention is further premised on the observation that, in today""s technology, there is an extremely high correlation between the commands transmitted to a camera and the camera""s actual physical movement. That is, a repetition of the same command, or set of commands, will result in the camera being positioned to the same field of view. Thus, it is not necessary to know the actual orientation angles and magnification factor corresponding to the camera""s field of view; it is sufficient to know the commands required to recreate the view. Because there is strong correlation and repeatability between commands and the actual camera field of view, a system in accordance with this invention can establish the correspondence of a particular mask to a particular view by storing the commands required to create, or re-create, the view.
In a preferred embodiment of this invention, the monitor will execute the appropriate commands to adjust the camera to produce a desired field of view, as viewed on the console. This particular view will be given a unique identifier, or address. The commands executed to move the camera to reproduce this view will be stored as the commands associated with this view. The monitor will then identify the areas selected for exclusion from motion detection; the corresponding mask will be stored as the mask image associated with this view. The monitor will repeat the above sequence for additional, uniquely identifiable, views. Thereafter, having identified specific views, and having stored the camera positioning commands and mask image associated with each view, the monitor need merely instruct the system to xe2x80x98go toxe2x80x99 a desired view. In response to this xe2x80x98go to viewxe2x80x99 command, the system will transmit the stored commands to adjust the camera to produce this view, then download the mask associated with this view. Note that, in this embodiment, coordinate transformations are not performed, and the camera field of view parameters, per se, are not required, thereby allowing for the use of conventional, low-cost, cameras and motion detectors.
In a further embodiment, motion detection can be automatically disabled while the camera is in motion, and automatically reenabled when the camera is positioned to the new view and the new mask is loaded.